Thousands of students in the Houston school district received good grades last year in Advanced Placement courses, but most didn't do well enough on the national AP exams to earn college credit.

The disconnect suggests widespread grade inflation, prompting HISD Superintendent Terry Grier to fire off an email to principals last week expressing his concerns.

District data show that students failed nearly two-thirds of the AP exams they took last school year while receiving A's or B's in the corresponding courses.

Grier said he was particularly troubled that students earned the lowest possible score on nearly 1,700 AP exams last year, yet received A's.

"This type of grade inflation is very difficult to defend," Grier wrote to the principals.

Understanding why more students are not passing AP exams - especially when they are making good grades - is key to the success of HISD's effort to prepare more students for college. The AP exams are standardized, so students in the Houston Independent School District take the same tests as others across the country.

Research shows that students who do well on AP exams do better in college than others, but researchers disagree about the value of AP courses when students score poorly on the exams.

Grier said he wants AP teachers to study the data for their own classes because there could be reasons other than grading practices for the discrepancy. Teachers may not make their assignments tough enough, for example. New to teaching AP classes, they may need more training.

Students may know the material but be poor test-takers, or lack motivation to try hard on a test that they didn't pay for and that doesn't affect their grades.

What's clear is that the AP exams, graded on a 5-point scale, are difficult. Colleges typically award course credits for scores of 3 or higher, although elite colleges have higher standards. A grade of 1 or 2 is considered a failure.

The College Board, the nonprofit that administers the AP program, does not set grading standards for high schools. But Trevor Packer, the organization's vice president over AP, said he would expect that students who scored at least a 4 on the exam would have earned an A in the class.

What about students with A grades and exam scores of 1 or 2?

"You shouldn't be that far out of alignment," Packer said.

Still, Packer said, HISD is not alone, as grade inflation plagues school districts and colleges across the country.

AP program grew fast

In HISD, most students who scored 4 or 5 on the AP exam earned A's and B's. The concern focuses on students who got those high grades but did poorly on the exams.

Teachers awarded nearly 13,000 A's and B's in AP courses last year, yet those students failed about 8,200 - or 63 percent - of the AP exams they took.

The exam scores, released in the summer, are not factored into students' grades.

Jason Catchings, who became principal of Scarborough High this school year, said he noticed a problem with grade inflation early on. He has been scraping his budget to train his teachers to discuss grading practices and make assignments more challenging.

"I'm not going to beat around the bush," Catchings said. "We had kids making high scores in the classroom but performing low on the AP test. That's an issue because colleges pick that up immediately."

Grier has led a rapid expansion of the AP program over the last two years, requiring high schools to offer at least 15 AP courses and encouraging schools to increase enrollment, especially among minorities and students from poor families.

Schools also are supposed to require students to take the AP exam if they take the course, and the district pays the test fees. Other districts may weed out weak students - which can inflate their AP passing rates.

"We put rigor in schools where kids weren't used to it," said HISD board president Mike Lunceford, who requested the AP grade analysis.

Praise for a teacher

Loyce Gayo, a senior at Worthing High School, said she deserves the grades she got in her AP classes last year - an A in English and a high B in U.S. history - yet she is ashamed of her low scores on the AP exams. She said she didn't feel prepared for the time limits.

Gayo said her exam scores didn't discourage her from enrolling in four AP courses this year, but she plans to study more and attend tutoring sessions.

The key to being prepared, said Davis High junior Cynthia Koboo, is hard work and a great teacher. Koboo earned a 3 on her AP world history exam last year and an A in the course.

"The class helped prepare me for the test a lot," she said. "We practiced writing all types of essays so many times. There's a lot more work involved, but it's more fun. You actually go into more depth than in regular classes."

Ruth Kravetz, a longtime AP calculus teacher at Davis, said some grade inflation may take place, but it's wrong to assume that students with low AP exam scores learned little and won't do well in college.

"It is possible," she said about grade inflation, "but it is as equally possible that the child worked really hard, learned a whole lot, demonstrated competency over the year and did not do well on the exam."